1. Field of the Invention
The present invention relates to a reference module apparatus of the type, for example, that serves to provide a reference voltage for performance of a measurement or detection, for example in a Root Means Square (RMS) detector circuit. The present invention also relates to a method of compensating for a base current of the type, for example, drawn by a reference circuit, such as a differential pair circuit, of an RMS detector circuit.
2. Description of the Related Art
In the field of optical equalisation, it is currently desirable to develop Integrated Circuits (ICs) to extend electrically the optical transmission distance achievable at 10 Gb/s over multi-mode fibre. A short-term goal is to achieve lossless data transmission at 10 Gb/s over 300 metres of multi-mode grade fibre. Currently, an equalizer architecture sufficiently robust to achieve the 300 metres of lossless transmission consistently is not available. Work is thus underway in a number of companies to improve the equalizer algorithms and their implementations in silicon and software to achieve this 300 meter goal.
In this respect, a post-amplifier Automatic Gain Control (AGC) circuit is employed as part of an integrated circuit to perform the above-described optical equalisation, the AGC circuit being provided for the purpose of so-called Electronic Dispersion Compensation (EDC). In some AGC circuits, a so-called RMS detector circuit is employed in order to detect a swing of a peak-to-peak input signal so as to be able to adjust a gain of the AGC circuit to maintain a peak-to-peak output signal of the AGC circuit within certain pre-defined limits.
Typically, the RMS detector circuit comprises two differential pair amplifiers coupled together, one operating in a difference mode and the other operating in a common mode. The differential pair circuit operating in the common mode is employed to generate a reference signal. However, due to the nature of bipolar transistors used in the common-mode differential pair circuit, base currents draw by the bipolar transistors introduce an error into the reference signal in the form of an offset voltage.
One known solution to the errors introduced is to replace the bipolar transistors with Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), since MOSFETs are known to possess high input impedances. However, for channel equalisation, maintenance of low offsets and high bandwidths is important. This is particularly true in respect of high-speed implementations of differential RMS detector circuits. In this respect, MOSFETs introduce greater offsets than bipolar transistors.
Additionally, the offset introduced by base currents of the bipolar transistors varies with temperature and process variation, i.e. the variation between instances of a given integrated circuit on different semiconductor wafers.